The present invention relates to an improvement of a toroidal-type continuously variable transmission used in vehicles such as automobiles.
A conventional toroidal-type continuously variable transmission is, as shown, e.g., in FIG. 6, constructed so that an input side disk 11 and an output side disk 12 are coaxially disposed so as to be opposed to each other inside a housing (not shown). An input shaft 13 passes through the shaft center of the toroidal transmission section having the input side disk and the output side disk. A loading cam 14 is disposed on an end of the input shaft 13. The loading cam 14 transmits the motive power (rotational force) of the input shaft 13 to the input side disk 11 through a cam roller 15.
The input side disk 11 and the output side disk 12, having substantially the same shape, are disposed so as to be symmetrical, and formed so as to be substantially semicircular in section as viewed in the axial direction with both opposed surfaces thereof taken into view. A pair of power rollers 16 and 17 that transmit motion are disposed so as to be in contact with the input side disk 11 and the output side disk 12, respectively, within a toroidal cavity formed by the toroidal surfaces of the input side disk 11 and the output side disk 12. Reference numeral 23 designates a thrust bearing.
In this case, the power rollers 16 and 17 are pivotably attached to trunnions 20 and 21 through pivots 18 and 19, and pivotably supported with a pivot O as the center, the pivot O serving as the center of the toroidal surface of the input side disk 11 and the output side disk 12. The surfaces of contact among the input side disk 11, the output side disk 12, and the power rollers 16 and 17 are supplied with a lubricating oil whose viscous frictional resistance is large, so that the motive power applied to the input side disk 11 is transmitted to the output side disk 12 through the lubricating oil film and the power rollers 16 and 17.
The input side disk 11 and the output side disk 12 are independent of the input shaft 13 (not being directly affected by the motive power of the input shaft 13) through needles 25. An output shaft 24 is attached to the output side disk 12. The output shaft extends in parallel with the input shaft 13 and is rotatably supported by the housing through an angular bearing 22.
In this toroidal-type continuously variable transmission the motive power of the input shaft 13 is transmitted to the loading cam 14. When the loading cam 14 is rotated by the transmission of the motive power, this rotational power is transmitted to the input side disk 11 through the cam roller 15, which in turn causes the input side disk 11 to rotate. The motive power generated by the rotation of the input side disk 11 is transmitted to the output side disk 12 through the power rollers 16 and 17. The output side disk 12 rotates integrally with the output shaft 24.
At the time of changing the speed, the two trunnions 20 and 21 are slightly moved toward the pivot O. That is, the axial movement of the trunnions 20 and 21 releases the intersection between the rotating shaft of the power rollers 16 and 17 and the shafts of the input side disk 11 and the output side disk 12. As a result, the power rollers 16 and 17 oscillate over the surfaces of the input side disk 11 and the output side disk 12, thereby changing the speed ratio to either accelerate or decelerate the motor vehicle.
Such a toroidal-type continuously variable transmission is disclosed, e.g., in Unexamined Japanese Patent Publication No. Sho. 63-203955 and Examined Japanese Utility Model Publication No. Hei. 2-49411. As conventional examples of the above-mentioned input side disk, output side disk, and power rollers, those using AISI52100 (equivalent of JIS SUJ2 high carbon chromium bearing steel) are known (see NASA Technical note, NASA ATN D-8362).
When driven, the above-mentioned toroidal-type continuously variable transmission generates among the input side disk, the output side disk, and the power rollers such a high contact stress as to reach a maximum contact stress of, e.g., about 4 GPa, which is so high a load as not to be received by usual rolling bearings and which is an inherent phenomenon in the toroidal-type continuously variable transmission. Therefore, there exists a problem in that the traction surfaces of the input and output side disks as well as the traction surfaces of the power rollers are susceptible to flaking. In addition, the bearing surfaces of the power rollers are put under a high contact stress, also causing such surfaces to flake in some cases.
To overcome this problem, efforts to prevent the flaking and fracture of the traction surfaces in the toroidal surface formed by the input and output side disks as well as the traction surfaces of the power rollers and to improve the reliability of these parts are called for.
Since the input side disk, the output side disk, and the power rollers are subject to repetitive bending stress, fatigue fracture tends to occur at these parts. Therefore, an improvement in fatigue strength under bending stress is also called for.
However, input and output side disks and power rollers for the conventional toroidal-type continuously variable transmission which satisfy the reliability of the traction surfaces and the bending stress life have not been achieved. That is, there is a need for studying not only the material of which the input side disk, the output side disk, and the power rollers are made, but also the heat treatment to be applied to such material.